This invention relates generally to radio frequency (RF) power amplifiers and, more particularly, the invention relates to an RF power amplifier having improved linearity and reduced intermodulation distortion.
RF amplifiers that operate outside of the small signal range are inherently nonlinear and thus introduce signal distortion when multiple signals are amplified. Recent requirements in cellular telephone technology have mandated increased capacity which in turn requires broad bandwidth, high dynamic range linear amplifiers where the output spectrum is free of unwanted intermodulation products. Intermodulation distortion within the RF amplifier passband can severely impede proper transmission and reception of cellular signals.
The concept of feedforward amplification to reduce distortion is known and has been successfully applied to RF amplifiers in the past. U.S. Pat. No. 5,157,345 and No. 5,334,946 disclose a feedforward amplifier which employs two signal cancellation loops in generating an error signal and then injecting the error signal as a feedforward correction signal. Copending application Ser. No. 08/369,546 filed Jan. 6, 1995, assigned to the present assignee, discloses a feedforward RF amplifier having a high dynamic range and low intermodulation distortion through use of closed loop control circuitry. Two control loops are employed with a main amplifier and an error amplifier. Copending application Ser. No. 098/412,479 filed Mar. 28, 1995, assigned to the present assignee, discloses a similar feedforward amplifier which employs a main amplifier for amplifying both the main signal and the error signal. In both applications, pilot signals are injected for use in determining amplitude and phase requirements for intermodulation distortion (IMD) cancellation.
Adaptive predistortion is another technique for error correction in RF power amplifiers. In effect, the input signal to a power amplifier is predistorted with intermodulation components inserted at the input of the RF power amplifier which correspond to the intermodulation components introduced by the power amplifier but of such phase and amplitude so that the intermodulation components cancel in the power amplifier.
The present invention is directed to an RF power amplifier with closed loop predistortion without the use of a pilot signal for error correction.